1. Field of the Invention
The present invention generally relates to methods of recording information on recording media such as rewritable optical discs, and particularly relates to an information recording method that records multi-value data on a recording medium by modulating the area size of recording marks.
2. Description of the Related Art
In respect of rewritable optical discs, there are product-to-product variations in optical discs and in optical disc recording/reproducing apparatuses, and also the varying conditions of use environment have some effect, so that the recording conditions need to be optimized with respect to each combination. In phase-change-type optical discs, the light emission period, recording power, and erasing power of a recording laser are controlled for the purpose of forming desired recording marks.
Conventional information recording methods are configured to achieve the uniformity of recording marks by controlling the recording power and/or recording pulse width according the linear recording velocity in order to achieve high-speed recording (see Japanese Patent Application Publication No. 10-106008, for example). Further, the recording conditions are optimized by feeding back the results of reproduction of recorded data to the recording pulse waveforms (see Japanese Patent Application Publication No. 2002-50046, for example).
The conventional information recording methods as described above are directed to setting optimum recording conditions in the case of binary recording. In binary recording, jitter worsens when a slice level for binarization changes (see FIG. 5 of Japanese Patent Application Publication No. 10-106008, for example). Because of this, the recording conditions are selected such as to set asymmetry within a predetermined value range, thereby optimizing the recording conditions. In multi-value recording, however, there is a need to suppress a deviation in the voltage direction (i.e., the signal level produced by marks) rather than a time deviation (i.e., the mark length or space length). In multi-value recording, therefore, the method of setting optimum recording conditions based on asymmetry as in the conventional information recording method cannot be used.
In the following, a description will be given of multi-value recording relating to the information recording method of the present invention.
FIG. 8 through FIG. 10 are drawings for explaining multi-value recording relating to the information recording method of the present invention.
As shown in FIG. 8, multi-value data is recorded separately for each recording cell that has a constant length, such that the level of a reproduced signal changes in response to the multi-value data. The reproducing of multi-value data is performed by sampling the reproduced signal at predetermined frequency (at the center position of each recording cell, for example) and by discriminating the multi-value data based on the level of the sampled signal. When a solitary wave that has no inter-code interference as recorded data is recorded for multi-value levels “0” through “7”, the level of the reproduced signal change linearly in response to the multi-value data as shown in FIG. 9, provided that the recording conditions are optimized.
If the recording conditions are not optimized, linearity deteriorates as shown in FIG. 9 due to too strong or too weak a recording power. Especially, a shorter recording mark (corresponding to multi-value data “1” through multi-value data “3”) is susceptible to the recording conditions and external influence. When the recording power is not sufficiently strong, for example, a gap between the threshold values for discriminating multi-value data narrows for multi-value data “1” through “3”, giving rise to a problem that the margin for reproduction decreases.
In order to achieve high-speed recording, the rate of disc revolution may be made constant, and the reference clock cycle of a recording signal may be changed according to radial position along the disc radius. Such high-speed recording is employed to record the CLV format in some recording media such as the rewritable DVDs.
When this method is applied to multi-value recording, recording is performed by changing the reference clock according to the linear recording velocity. Since the recording sensitivity of an optical disc is not linear with respect to linear recording velocity, however, the shorter marks (corresponding to multi-value data “1” through “3”) tend to be formed larger as shown in FIG. 10. Namely, the relationship between the multi-value data and the signal levels varies according to the linear recording velocity, which creates a need to learn a multi-value discrimination level and/or waveform equalization coefficient separately for each linear recording velocity. At the time of reproduction, the multi-value discrimination level and waveform equalization coefficient corresponding to the linear velocity are retrieved for reproduction, which gives rise to a problem that reproduction becomes time consuming.
In consideration of the above, there is a need in multi-value recording to achieve stable recording regardless of changes in ambient conditions and product-to-product variation in recording media such as optical discs and information recording apparatuses such as optical disc apparatuses for recording information on such recording media.
Also there is a need in multi-value recording to achieve information recording that allows a substantially constant reproduced signal level to be obtained over the entire surface of a recording medium when multi-value data is recorded by the CAV method with constant linear recording density.